[Illustration: FIG. 23.—Ideal, vertical and sagital section, representing the ontogeny and phylogeny of the human brain. of, olfactory lobe; crf, cerebrum of fish; ol, optic lobes of fish; cbf, cerebellum of fish; m, medulla of fish; cbr, cerebellum of reptile; cbo, cerebellum of opossum; cbl, cerebellum of lemur; cbm, cerebellum of man; cr, cerebrum. Cerebrum convoluted in lemur; much more convoluted in man. Cerebellum convoluted from opossum upwards; mm, medulla of man. Modified from Le Conte.] It is a very interesting and instructive fact that in the development of the human brain from the fertilized ovum these same stages, which are permanent conditions in the zoölogical (taxonomic) series, are passed through by it as transient stages. One of the earliest conditions of the human brain is that in which it presents three swellings in a serial arrangement. They are known from behind, forwards as hindbrain, midbrain, and forebrain. For our purposes it is sufficiently accurate to say that the =fœtal brain=, in developing from this early condition to a later and higher condition, differentiates the hindbrain into the _medulla_ (Fig. 24, m) and the _cerebellum_ (cb); the midbrain becomes the _optic lobes_ (ol); and the forebrain differentiates into the _thalami_ (th) and the _cerebrum_ (cr). A little later the cerebrum buds forth the olfactory lobes (of), so that the human brain will consist of six fundamental segments,—one behind the other. This is the _fish stage_ in the growth of the human brain. (Compare Fig. 24 with Fig. 19.) [Illustration: FIG. 24.—Diagrammatic representation of the brain of a human fœtus of the third week. Representing the fish-phase in the ontogeny of the human brain. Side view. cr, cerebrum; th, thalamus; ol, optic lobes; cb, cerebellum; m, medulla. The olfactory lobes at this stage are very small and are not shown. FIG. 25.—Dorsal view of the brain of a human fœtus of about seven weeks. Representing the reptilian phase in the ontogeny of the human brain. cr, cerebrum; th, thalami; ol, optic lobes; cb, cerebellum; m, medulla. FIG. 26.—Side view of the brain of a human fœtus of about three months. Representing the marsupial phase in the ontogeny of the human brain. cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla. The thalami are completely, and the optic lobes partially, covered by the greatly enlarged cerebrum.]